$\sin(-\frac{9}{4}\pi) = \text{?}$
To find the sine using the unit circle, first find the angle. Drag the orange point around the circle until $-\frac{9}{4}\pi$ is selected. The correct angle is selected. Remember, the sine of an angle is represented by the $y$ coordinate of a point on the unit circle. The $y$ coordinate of the point is $-0.707$, so $\sin(-\frac{9}{4}\pi) = -0.707$.